Massive Stars in the Far and Extreme Ultraviolet

TL;DR

This paper discusses the importance of accurate spectral energy distribution models for massive stars, emphasizing the complexities in estimating their ionizing fluxes in the far and extreme UV regions due to line-blanketing effects.

Contribution

It highlights the necessity of advanced atmosphere models that include line-blanketing to accurately estimate ionizing fluxes of massive stars.

Findings

Line-blanketing significantly alters the SEDs of hot stars.

Accurate models are essential for understanding ionization in astrophysical environments.

Deviations from blackbody spectra are substantial in the UV region.

Abstract

From the main sequence to their late evolutionary stages, massive stars spend most of their life as hot stars. Due to their high effective temperatures, the maximum of their emitted flux falls into the ultraviolet (UV) regime. Consequently, these stars emit a significant number of photons with energies sufficiently high enough to ionize hydrogen and other elements. As simple as these fundamental considerations are, as complex is a realistic estimate of the resulting ionizing fluxes, in particular for energies above 54 eV. Estimating the ionizing flux budget of hot stars requires accurate models of their spectral energy distributions (SEDs), covering in particular the far and extreme UV region. Modern atmosphere models that incorporate the so-called line-blanketing effect, i.e. taking into account the millions of lines from iron and other elements, yield a complex picture, illustrating that the SED of a hot, massive star usually deviates significantly from a blackbody.